Method and apparatus for recording three-dimensional distribution of light backscattering potential in transparent and semi-transparent structures

Abstract

An apparatus is disclosed for generating data representative of a three-dimensional distribution of the light backscattering potential of a transparent or semi-transparent object such as a human eye. The apparatus includes an interferometer, both the reference beam and measurement beam of which are directed toward the object and reflected by respective reference and measurement sites thereof, such that axial motion of the object during measurement affects both beams equally. The measurement beam is raster scanned transversely across each measurement site for which data is obtained. Also, the frequency of one of the beams is shifted by a non-moving frequency shifter, such that the reflected beams combine and are modulated by a heterodyne beat frequency, which is detected when the object path difference is matched with the interferometer path difference. Because the non-moving frequency shifter can effectively generate a beat frequency of about 40 MHz, relatively rapid transverse and longitudinal scanning are facilitated.

What is claimed is:  
     
       1. An apparatus for matching an object path difference with an interferometer path difference and identifying the matching by heterodyne detection of a reference beam and a measurement beam, comprising: 
       a single light source of the reference beam and a measurement beam; and  
       means for shifting the frequency of at least one of: the reference beam, and the measurement beam, by at least ten megaHertz, to thereby generate a beat frequency when the object path difference bears a predetermined relationship to an interferometer path difference.  
     
     
       2. The apparatus of claim  1 , wherein the means for shifting is at least one non-moving frequency shifter positioned in at least one path of the light beams, and the apparatus further comprises: 
       an interferometer through which the beams propagate;  
       a receiver receiving the reflected beams and generating a signal representative thereof, the signal being usable for determining the distribution of the light backscattering potential in the object, the reference beam being reflected by a reference surface defined by the object and the measurement beam being reflected by a measurement site defined by the object, such that axial motion of the object relative the interferometer equally affects the reference beam and measurement beam.  
     
     
       3. The apparatus of claim  1 , wherein the means for shifting is selected from the group of frequency shifters including acousto-optic frequency shifters, and phase modulators. 
     
     
       4. The apparatus of claim  1 , wherein the apparatus defines at least a reference arm and a measurement arm, and the apparatus includes at least one translationally movable mirror or retroreflector to selectively establish the length of at least one arm. 
     
     
       5. The apparatus of claim  1 , wherein the apparatus defines a light entrance path and a light exit path, the means for shifting being positioned in one of the arms, the apparatus further including: 
       a polarizer in the light entrance path;  
       a quarter wave plate in at least one of the arms; and  
       a half wave plate positioned in the arm in which the means for shifting is positioned.  
     
     
       6. The apparatus of claim  1 , further comprising: 
       at least one dispersion compensation element disposed in at least one of the beams.  
     
     
       7. The apparatus of claim  1 , wherein the apparatus defines at least a measurement arm, and the apparatus further comprises a scanning mirror positioned in the measurement arm and tiltable in at least one degree of freedom to selectively establish a direction of propagation of a light beam impinging on the scanning mirror. 
     
     
       8. The apparatus of claim  7 , further comprising a coherence layer correcting element juxtaposed with the scanning mirror to alter a length of a path traversed by the measurement beam in proportion to an angle established between the measurement beam and a null direction for the measurement beam. 
     
     
       9. A method for generating data representative of a three-dimensional distribution of the light backscattering potential of an object defining at least one reference surface and plural measurement sites, comprising: 
       directing a reference beam from a reference arm of an interferometer against the reference surface;  
       directing a measurement beam from a measurement arm of the interferomneter against the measurement site;  
       shifting the frequency of at least one beam by at least ten million Hertz;  
       combining reflections of the beams from the surfaces; and  
       detecting the combined reflections.  
     
     
       10. The method of claim  9 , further comprising the step of altering a length of a path traversed by the measurement beam in proportion to an angle established between the measurement beam and a null direction for the measurement beam. 
     
     
       11. A method for detecting a distributional profile of a light backscattering potential, comprising: 
       (a) generating light beams with short coherence length;  
       (b) scanning at least one of the light beams in at least one dimension with at least one scan mirror;  
       (c) shifting the optical frequency of at least one of the light beams with at least one non-moving frequency shifter;  
       (d) directing the light beams towards the object, such that at least one of the light beam is directed at a reference site of the object, and at least one of the light beams is directed at a measurement site of the object; and  
       (e) directing the reflected light beams from the object to a receiver and generating a signal representative thereof, the signal being usable for determining a multidimensional distribution of light backscattering potential in the object, wherein the plural light beams include at least one reference beam directed at the reference site and at least one measurement beam directed at the measurement site, such that axial motion of the object relative to the incident direction of the light beams to the object equally affects the reference beam and the measurement beam.  
     
     
       12. The method of claim  11 , wherein the signal generated at the receiver has a beat frequency when the traveling distance of the reference beam and the traveling distance of the measurement beam are substantially matched. 
     
     
       13. The method claim  11 , further comprising changing the path length of at least one of the light beams by moving at least one element in a path delay unit. 
     
     
       14. The method claim  11 , further comprising inserting at least one polarizing beam splitter and at least one quarter wave plate in the light beam paths, such that the reflected light beam shall be directed to avoid passing through the frequency shifter element in its return path. 
     
     
       15. The method claim  11 , further comprising inserting a spatial filter in at least one of the light beam path. 
     
     
       16. The method claim  12 , further comprising controlling the pointing direction of the scan mirror in at least one dimension and the step of controlling the difference of the traveling path distances of the reference beam and the measurement beam, such that signal can be generated at the receiver at each predetermined location of a three dimensional backscattering potential of the object. 
     
     
       17. The method of claim  11 , further comprising inserting at least one beam path compensator element in the measurement beam path such that the signal correspond to a coherence layer of the backscattering potential is a curved surface which has substantially same curvature of a retina when the measurement beam is scanned by tilting at least one of the scan mirror while fixing the position of the path delay unit. 
     
     
       18. The method of claim  17 , further comprising selecting the beam path compensator which can be chosen from a list including: a glass plate with plane parallel surfaces, or a pair of movable wedges which forms a substantially plane parallel surfaces with adjustable optical thickness. 
     
     
       19. An apparatus for matching an object path difference with an interferometer path difference and identifying the matching by heterodyne detection of a reference beam and a measurement beam, comprising: 
       a single interferometer through which the reference beam and measurement beam propagate; and  
       means in the interferometer for shifting the frequency of at least one of: the reference beam, and the measurement beam, by at least ten megaHertz, to thereby generate a beat frequency when the object path difference bears a predetermined relationship to an interferometer path difference.  
     
     
       20. The apparatus of claim  19 , wherein the means for shifting is at least one non-moving frequency shifter positioned in at least one path of the light beams, and the apparatus further comprises: 
       a receiver receiving the reflected beams and generating a signal representative thereof, the signal being usable for determining the distribution of the light backscattering potential in the object, the reference beam being reflected by a reference surface defined by the object and the measurement beam being reflected by a measurement site defined by the object, such that axial motion of the object relative the interferometer equally affects the reference beam and measurement beam.  
     
     
       21. The apparatus of claim  19 , wherein the means for shifting is selected from the group of frequency shifters including acousto-optic frequency shifters, and phase modulators. 
     
     
       22. The apparatus of claim  19 , wherein the apparatus defines at least a reference arm and a measurement arm, and the apparatus includes at least one translationally movable mirror or retroreflector to selectively establish the length of at least one arm. 
     
     
       23. The apparatus of claim  19 , wherein the apparatus defines a light entrance path and a light exit path, the means for shifting being positioned in one of the arms, the apparatus further including: 
       a polarizer in the light entrance path;  
       a quarter wave plate in at least one of the arms; and  
       a half wave plate positioned in the arm in which the means for shifting is positioned.  
     
     
       24. The apparatus of claim  19 , further comprising: 
       at least one dispersion compensation element disposed in at least one of the beams.  
     
     
       25. The apparatus of claim  19 , wherein the apparatus defines at least a measurement arm, and the apparatus further comprises a scanning mirror positioned in the measurement arm and tiltable in at least one degree of freedom to selectively establish a direction of propagation of a light beam impinging on the scanning mirror.